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Sedimentary distribution and characteristics near Dokdo

HOME > Marine Science Reports > Sedimentary distribution and characteristics near Dokdo
Dokdo is an island in the form of a guyot left after the erosion of a submarine mountain that appeared as a result of volcanic action 4.6 million to 2 million years ago. Volcanic stratification led to a strata structure in Dokdo where the bottom layer is trachyte I, followed by massive (tuff) breccia, stratified tuff, trachyandesite, scoria-like welded tuff, trachyte II, trachyte III and trachyte dyke. Some exposed trachyte has been found in some northern areas of Dongdo (East Island) and some southeastern coastal areas of Seodo (West Island).

The marine deposits near Dokdo consist mainly of volcanic rock and volcanic clastic rock fragments originating from land and carbonate sediments produced by invertebrates and plants living in shallow seas. Carbonate sediments consist mostly of mollusks (bivalvia and gastropods), encrusting bryozoans, encrusting red algae, worm tubes and echinoderms which originate in shallow waters but pile up along the submarine topography of Dokdo - along both the gentle slope that continues from the coast to a depth of 90 to 175 meters and the steep slope that plunges from a depth of 200 to 2000 meters.

A particle-size analysis of surface sediments near Dokdo shows various particle sizes, from mud to muddy sand, muddy sandy gravel, sandy gravel and gravel. Although water depth is the major determinant of particle size and components in marine sediments near Dokdo, the flow pattern of currents and geographical features can cause discrepancies in different areas of similar depths. At a depth of up to 200 meters, most marine deposits consist of sandy sediments with small amounts of gravel, while some areas have a more gravelly type of deposit. At depths that are greater than 2,000 meters, marine deposits are mostly muddy. The pathway through Ulleung Basin that links Dokdo and Ulleungdo also consists mostly of mud sediments. In areas closer to Ulleungdo, water depth becomes shallower, and sand sediments with small amounts of gravel become the main component of marine deposits.

The marine deposits near Dokdo are characteristic of cool water carbonate sediments, and the facies changes as follows according to the water depth. (Figure 1)
  • 1. Nearshore facies (shallower than 20 m)
  • 2. Neritic facies (at a depth of 20 - 100 m)
  • 3. Upper transitional facies (at a depth of 100 - 200 m)
  • 4. Lower transitional facies (at a depth of 200 - 700 m)
  • 5. Hemipelagic facies (deeper than 700 m)
Figure 1. Sedimentary facies by water depth near Dokdo
Figure 1. Sedimentary facies by water depth near Dokdo
Nearshore facies and neritic facies (shallower than 100 m) mostly consist of carbonate sediments formed by a variety of shallow-water dwelling organisms that secrete calcareous skeletons. The neritic facies usually consist of planktonic foraminifera, sponge spicules and volcanic rock fragments, and there is a relatively small amount of sediment here that originates from the same shallow seas. Some difference in sediment components due to the different sediment supply can also be observed. The transitional facies (at a depth of 100 - 700 m) mostly consists of planktonic foraminifera and sponge spicules with small amounts of clay minerals. Volcanic rock fragments are sometimes found in some areas of the upper transitional facies, but not in the lower transitional facies. The hemipelagic facies (deeper than 700 m) consist mainly of clay minerals. Here, as is typical of pelagic sediments, a small amount of planktonic foraminifera will be found in sediments comprised mostly of mud that originated on land. Sediments that were sampled below a depth of 2,000 meters still contain a significant amount of carbonates (ca. 13-23%), implying that the carbonate compensation depth (CCD) in the seas of Dokdo may well exceed this water depth. (Figure 2)
Figure 2. (a) Nearshore facies sediments (b) Neritic facies sediments (c) Transitional facies sediments (d) Hemipelagic facies sediments
Figure 2.
(a) Nearshore facies sediments (b) Neritic facies sediments
(c) Transitional facies sediments (d) Hemipelagic facies sediments
It is quite interesting to see that marine deposits near Dokdo mostly consist of carbonate sediments - especially since earlier studies showed that carbonate sediments usually appear in tropical and sub-tropical seas where temperatures are high and the turbidity is low. More recent studies on higher latitude shallow water sediments have reported that carbonate sediments also appear in shallow water environments in the temperate zones or at even higher latitudes, and have called such sediments ‘cool water carbonates’. Cool water carbonates have been found in the southern continental shelf of Australia, Baja California and in New Zealand. It was thought that the distribution of cool water carbonates was closely related to the distribution of coral reefs and was mainly affected by temperature, but recent studies suggest that cool water carbonates are influenced not only by temperature, but also by the inflow of clastic particles, oceanographic conditions, the concentration of nutrients and the sea-level changes.

Cool water carbonates have also been identified near Jejudo and Yeosu’s Sado. The beach sediments found in Jejudo consist of clastic particles originating from volcanic action combined with carbonate particles from mollusks and red algae. The amount of carbonate particles found in the sediment seems to depend on the distribution of rivers and streams that carry down volcanic rock fragments. Jejudo also has carbonate dune deposits that are created as carbonate beach sediments are blown to the dunes by the wind. Carbonate sediments have also been discovered in the shallow waters of Dokdo. This is the first report of cool water carbonate deposits in the mid- to high- latitudes of the Pacific Ocean in the Northern Hemisphere.